The present invention relates to a system for controlling transmission ratio and drive speed for a motor vehicle, and more particularly to a system for decelerating the vehicle during a constant speed drive.
In a control system for a constant speed drive (hereinafter called a cruise control system) for a motor vehicle having an automatic transmission, when coasting, that is, deceleration of the vehicle, is selected by the driver, the vehicle is decelerated by reducing the opening degree of a throttle valve without downshifting of the transmission. In an ordinary automatic transmission, the transmission ratio is stepwisely changed. Accordingly, if the transmission is downshifted during the coasting, the vehicle is excessively decelerated with a shock, thereby impairing the driving characteristic. Thus, it is preferable to decelerate the vehicle by closing the throttle valve.
Japanese patent application Laid-Open No. 60-135335 discloses a cruise control system for a motor vehicle with a continuously variable belt-drive automatic transmission (CVT) wherein a current control means is provided for controlling current passing to a vacuum operated throttle actuator in accordance with the difference between a set vehicle speed Vs and an actual vehicle speed V. A correcting means is provided for correcting transmission ratio i for the transmission in accordance with current or a control signal for the current control means, thereby controlling the actual vehicle speed V to the set vehicle speed Vs.
However, the deceleration rate of the vehicle speed depends solely on the reduction rate of the engine torque. Thus, the vehicle is slowly decelerated. Accordingly, the driver, feeling a dangerous delay of the deceleration, depresses a brake pedal, which causes cancelling of the cruise control. In order to resume the cruise control, the setting operation must be done.